Yep. Would be quite an interesting argument to make that removing a file serves as an effective means of unilaterally rescinding a perpetual patent license.
This is actually a concern of mine as well.... that for the user, facebook’s act of removing the patents file could be worse than if the patents file was never even there to begin with. That removing the file doesn’t revert it to potentially implicit, but instead is seen as explicitly revoking. But I don’t think that’s how it should go, especially since they also switched over to MIT license in the process. But I’m also not a lawyer and probably have zero credibility writing about any of this.